Abstract: Based on the daily NCEP reanalysis data (1º×1º) in four times and the conventionally observational data, an urban snowstorm process on March of 2011 in the middle of Tianshan mountains of Xinjiang was diagnosed. The diagnostic study includes a mesoscale analysis, calculations of moisture flux and its divergence, horizontal divergence, vertical speed, upper and lower wind fields, helicity and pseudo-equivalent potential temperature. The results indicate that the snowstorm is caused by the eastward movement of the north and south frontal zones after joining up in the central Asia region. The strong southeast wind occurs before snowfall at the urban area of Urumqi. The sharp decompression and warming on the ground provide thermodynamic conditions for the instability energy. There is the southwesterly jet whose wind speed is greater than 30 m·s^-1 at 500 hPa, and there exists low-level shear lines at 700 hPa. The vertical distribution of divergence and vertical velocity presents obvious symmetrical structure of lower-level convergence and upper-level divergence. In the period of heavy snowstorm, it shows a couple of vertical helicity with upper negative and lower positive. The low energy tongue of θ_se extends upward to 200 hPa, while the high energy tongue of θ_se maintains from 700 hPa to 400 hPa. The deep moist layer reaches to 300 hPa. This kind of configuration of the physical quantity field accelerates the convergence of wet air in low level and their lifting movement upwards, which provides the necessary conditions for generation of snowstorm. The cold air influences the urban area of Urumqi along a western path. Under the cold air condition, the strong snowstorm usually occurs at the central urban areas with higher temperature due to the effect of urban heat island．Meanwhile, the rainfall center is prone to form in the location of frontal zone.

Key words: font-family: "Times New Roman", mso-font-kerning: 1.0pt, mso-ansi-language: EN-US, mso-fareast-language: ZH-CN, mso-bidi-language: AR-SA, Snowstorm')">mso-fareast-font-family: 宋体">Snowstorm, Helicity, Water vapor flux